1. Field of Invention
The invention relates to a digitizing device with a digitizing tablet having, for each of the two coordinate directions, a first grid winding divided into regularly repeated strip conductor groups and a second grid winding divided into regularly repeated strip conductor groups. These first and second grid windings are arranged in such a way that the first strip conductors of adjoining strip conductor groups of these first and second grid windings have different spacings in the associated coordinate direction over the entire work surface. A predetermined sequence of periodic signals is applied to the grid windings for the one coordinate direction by a trigger circuit. For a predetermined length of time, a first signal is applied to the first end of the first grid winding; and for an immediately following second predetermined period of time, a second signal is applied to the first end of the second grid winding. Immediately following the triggering of the grid windings for the one coordinate direction, the grid windings for the other coordinate direction are triggered in the same manner. The device also includes a scanning coil which can be placed on the digitizing tablet. The signals induce voltages in the scanning coil which are supplied to an evaluating circuit for detecting the position of the scanning coil on the digitizing tablet.
2. Brief Description of the Prior Art
In a known digitizing device of this type (German Patent DE-PS 35 44 673), one of the grid windings is used for performing a rough measurement, and the other is used for performing a fine measurement. The grid windings are triggered with periodic digital pulse sequences, so that during the digitizing operation signals are always applied to one of the grid windings. The trigger circuit and the evaluating circuit thereby cooperate in such a way that the evaluating circuit directly receives information relating to the start of the application of a signal to the respective grid winding. Thus, from the phase shift of the crossovers it is possible to detect the strip conductors of the first grid winding in the area of which the scanning coil is located. This phase difference represents a defined value determining the respective strip conductor group of the first grid winding, because the distances between the strip conductor groups of the first and second grid windings are different. The evaluating circuit determines an exact position, within the previously determined strip conductor group of the second grid winding, from the crossover of the signal of the second grid winding, and thus determines an exact position of the scanning coil upon the work surface for a first coordinate direction. A determination of the position of the scanning coil in the second coordinate direction upon the work surface is performed in the same fashion.
The known digitizing device operates extremely satisfactorily and exactly, but requires the direct cooperation of the trigger circuit and the evaluating circuit. Thus, the digitizing tablet and the scanning coil must essentially represent a connected component.